I browsed Area51 looking for a quotation I seem to remember existed but cannot find, unfortunately. However, I came across this blog post, which nicely states the problem, and this later one in which the SE team’s proposed solution is described.
Scandinavian languages have been proposed at least once. If we consider the four relevant criteria, we find that we already fail at the first one: No question on Scandinavian is on-topic on German.SE as of now. We also fail at 2 (no tag for Danish/Swedish/Norwegian/Faroese/Icelandic/Old Norse). I assume number 3 to also fail (I believe the critical mass of speakers of Scandinavian languages to be high enough on the long run), and finally we fail absymally at 4 — hardly any German speaker would enjoy seeing the occasional question of or in, say, Swedish.
It can be argued whether we fail 1 and 2 due to our narrow choice or due to other, more generic reasons. However, the points raised by Wrzlprmft are pretty much spot on. Swedish and Danish are pretty well understandable by speakers of the corresponding language but neither of them is understandable by German natives per se. Contrary to that, quite a few people say that they are able to understand Dutch if spoken slowly, and I have had the occasional Flemish Belgian tell me that they understood my Southern German well enough. This actually can be explained by the languages’ histories: Proto-Germanic split into North Sea Germanic and continental Germanic before today’s subset of languages developped. North Sea Germanic would later diverge into the Scandinavian languages, while continental Germanic was the predecessor of German and Dutch. Until 1648, the Netherlands belonged to the Holy Roman Empire and thus to the construct that would most likely be labelled Germany, and if I am not mistaken, throughout the middle ages the difference between the language spoken in Antwerpen and that spoken in Vienna would have been, by modern standards, considered a single dialect continuum. All of this means that defining Scandinavian languages as off-topic is rather well-based.
(Note, by the way, that English is closer to the Scandinavian branch, possibly due to the Viking conquests, although this is most likely debated — and I am not a linguist, I can only repeat what I read in the paper.)
Now let’s turn our attention back to Linux/Ubuntu (the first link). We need to ask ourselves:
Whether German speakers would be happy to be classified as Germanic language speakers, like to be lumped together with those and happy to accept questions about Danish, Norwegian, etc. and
whether speakers of Scandinavian languages would be happy to be classified as Germanic language speakers, like to be lumped together with those and happy to accept questions about German.
I suspect neither. As outlined above, there is no mutual intellegibility between the Scandinavian group and German. Thus, we would effectively create two hardly intermangling niches on a single site. Furthermore, it would be a lot easier to gain ‘German’ reputation than ‘Scandinavian’ reputation, since the overwhelming proportion of questions would still be about German (approximately 100 million speakers worldwide compared to 10 million Swedish, 5 million Danish, 5 million Norwegian, 300.000 Icelandic and less than 100.000 Faroese speakers. Dutch omitted on purpose — it would only make matters worse for the Scandinavians.) which would lead to a lot of frustration by the Scandinavians. Community-voted Scandinavian question closure would be a big problem. Also, German speakers may well misinterpret Germanic and wonder why the occasional Scandinavian question is there (and probably be irritated by it).
As a final point, I remember a discussion on Area51 that seems to have been deleted along with the Polish language proposal, about how good an idea it is to mix Polish and/or Russian with the Slavic languages proposal. There was unanimous agreement that Russian should not be included since it would give a huge bias of Russian speakers meaning all other languages would be lost. But there was also a rather general agreement that even Polish should probably be excluded since Polish is the language with the largest group of speakers. (Polish: 40 million, Ukranian: 32 million, Czech: 13 million, Bulgarian: 10 million — why the argument did not extend to Ukranian I cannot tell you.)